When exposed to chronic sublethal hypoxia the developing brain responds with increases in permeability and angiogenesis. Vascular endothelial growth factor (VEGF) may mediate this response. Here, we present data on the localization of VEGF in the rat brain cortex during postnatal development and its correlation to vascularization. We reared newborn rats under normoxic conditions and in hypoxic chambers (FiO(2) 9.5%), removed them at postnatal days (P) 3, 8, 13, 24, and 33 and prepared the cortical brain tissue for immunohistochemistry, in situ hybridization (ISH), Western blot analyses and vessel density counting. When compared to age-matched controls, hypoxic-reared animals displayed a significant increase in platelet endothelial cell adhesion molecule 1 (PECAM-1) protein levels, cerebral microvascular lumen diameter and number and density of vessels (number of capillaries per area). In control animals, ISH and immunohistochemistry revealed that localization of VEGF is restricted almost exclusively to cortical neurons at early stages of development. As the vascular bed begins to stabilize, predominant VEGF expression switches to maturing glial cells which invest vessels while neuronal expression is reduced to a basal level. In hypoxic animals, early localization of VEGF is also restricted to cortical neurons, however, during later developmental stages, glial cells express elevated levels of VEGF protein and high neuronal expression also persists. Thus chronic sublethal hypoxia disrupts the temporal-spatial expression of VEGF, which correlates with continuing hypoxia-driven angiogenesis.